Electron guns with magnetic focusing



Oct. 20, 1959 o. HEIL ELECTRON GUNS WITH MAGNETIC FOCUSING Filed Oct. 17, 1958 INVENTOR.

N a m T f H m United States The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This invention relates to an improved electron gun which makes use of a specially shaped focusing magnetic field to increase the electron density Within an electron beam.

One object of the invention is to provide an electron gun which will give increased current densities and which will therefore increase tube life and tube reliability by decreasing the specific surface load of the cathode.

This and other objects will be more fully understood from the following detailed description taken with the drawing wherein:

Fig. 1 is a longitudinal cross sectional view of a reflex klystron using an electron gun of the invention.

Fig. 2 is a cross sectional view of an electron gun according to a further embodiment of the invention.

Additional focusing is obtained according to the invention by the application of a converging magnetic field. Two different arrangements are shown for providing this additional focusing. In the first, the converging magnetic field is formed within the cathode space so that a magnetic equipotential surface coincides with the emitting surface of the cathode and in the second arrangement the converging magnetic field is outside of the cathode space.

With the gun shown, the electron density is greater at the edge of the beam than near the center and there therefore results a greater space charge lens on the edge of the beam. A gun of this type is shown in my Patent No. 2,518,472. The converging magnetic field produces a greater focusing effect upon the outer electrons of the beam and therefore compensates for the greater dispersing space charge lens at the edge of the beam.

Referring more particularly to Fig. 1 of the drawing, reference numeral refers to a reflex klystron having a hot cathode 11, a cold cathode 12, accelerating electrode 13 and a resonant cavity 17. A magnetic field is produced between the disc 14 and the pole piece 15 which also acts as a repeller electrode. The magnetic field is provided by a permanent magnet 16. A coil can be substituted for permanent magnet 16 if desired. A ring of magnetic material 18 is located behind the cathode to correct the magnetic field, which penetrates through the opening in the disc 14, and causes the magnetic lines of force to leave the cathode perpendicular to the surface. The relative size and position of the ring 18 should be substantially the same as that shown in the drawing. In this embodiment, the converging magnetic field is formed within the gun structure and the beam is focused only once.

Fig. 2 shows how the converging magnetic field can be formed outside of the cathode space. In this figure, hot cathode 11, cold cathode 12 and accelerating electrode atent O 2,909,691 Patented Oct. 20, 1959 ICC 13 corresponds to like electrodes in Fig. 1. Pole piece 24 is cone shaped and has an axial cone shaped opening 25 machined therein. A converging magnetic field is established between the inner surface of pole piece 24 and a pole piece 26, by means of a permanent magnet 31. A coil may be provided in place of the permanent magnet 31 and pole piece 26. A narrow neck 27 in the pole piece 24 prevents the magnetic field from penetrating into the accelerating space of the gun. A small auxiliary permanent magnet 28 may be provided to further prevent the disturbing efiect of a penetrating magnetic field. The electron beam leaving the gun is a divergent beam due to its own space charge repulsion. This beam is then refocused by the converging magnetic field between pole pieces 24 and 26 so that a beam of high density leaves the nozzle 29 in member 30. Member 30 is made of a nonmagnetic material such as copper. Higher density increases in the beam have been achieved with the gun structure ofFig. 2 than with the gun structure of Fig. 1.

While the gun of the invention has been shown as used in a reflex klystron, it is to be understood that it can be used with other type tubes such as travelling wave tubes.

There is thus provided an electron gun which will give increased current densities and which will increase tube life and tube reliability.

While certain specific embodiments have been described in some detail, it is obvious that numerous changes may be made without departing from the general principles and scope of the invention.

I claim:

1. An electron gun comprising; means for producing a beam of electrons having a higher density on the edge than on its axis; a conical shaped pole piece having its apex pointing toward said beam producing means; a ferromagnetic disc, having an opening therein, adjacent said beam producing means; an accelerating electrode located within said opening; means for producing a converging magnetic field between said pole piece and said disc; and means, on the side of said beam producing means remote from said accelerating electrode, for correcting the magnetic field that penetrates through the opening in said disc to make the magnetic lines of force leave the beam producing means perpendicular to its surface.

2. An electron gun structure, comprising; means for producing a beam of electrons having a higher density on the edge than on its axis; a ferromagnetic disc adjacent said beam producing means; said disc having a conical shaped projection thereon; said disc having an opening consisting of two oppositely disposed tapered surfaces meeting at a narrow neck with the portion of the opening remote from said beam producing means having a substantially conical shape; means including said disc for producing a converging magnetic field within the remote portion of said opening; said disc acting to accelerate said beam through said converging mag netic field; and field compensation means located, on the side of said beam producing means, remote from said disc.

References Cited in the file of this patent UNITED STATES PATENTS 2,306,875 Frernlin Dec. 29, 1942 2,608,668 Hines Aug. 26, 1952 2,615,141 Hansell Oct. 21, 1952 2,619,611 Norton et a1. Nov. 25, 1952 2,632,130 Hull Mar. 17, 1953 2,829,299 Beck Apr. 1, 1958 

